The present invention relates to spinal fixation devices and more specifically relates to a pedicle screw assembly having a pedicle screw with an expandable head In a preferred embodiment, the assembly includes a coupling element having a seat for receiving the head of the screw and allowing polyaxial movement of the screw relative to the coupling element, and an insert for expanding the head of the pedicle screw against the seat of the coupling element so as to prevent movement of the coupling element and pedicle screw relative to one another.
The spinal column is a highly complex system of bones and connective tissues that provides support for the body and protects the delicate spinal column and nerves. The spinal column includes a series of vertebrae stacked one atop the other, whereby each vertebral body includes a relatively strong bone portion forming the outside surface of the body (cortical) and a relatively weak bone portion forming the center of the body (cancellous). Situated between each vertebral body is an intervertebral disc that provides for cushioning and dampening of compressive forces applied to the spinal column. The vertebral canal containing the delicate spinal cords and nerves is located just posterior to the vertebral bodies.
Various types of spinal column disorders are known and include scoliosis (abnormal lateral curvature of the spine), kyphosis (abnormal forward curvature of the spine, usually in the thoracic spine), excess lordosis (abnormal backward curvature of the spine, usually in the lumbar spine), spondylolisthesis (forward displacement of one vertebra over another, usually in a lumbar or cervical spine) and other disorders caused by abnormalities, disease or trauma, such as ruptured or slipped discs, degenerative disc disease, fractured vertebra, and the like. Patients that suffer from such conditions usually experience extreme and debilitating pain as well as diminished nerve function.
The present invention involves a technique commonly referred to as spinal fixation whereby surgical implants are used for fusing together and/or mechanically immobilizing adjacent vertebrae of the spine. Spinal fixation may also be used to alter the alignment of the adjacent vertebrae relative to one another so as to alter the overall alignment of the spine. Such techniques have been used effectively to treat the above-described conditions and, in most cases, to relieve pain suffered by the patient. However, as will be set forth in more detail below, there are some disadvantages associated with current fixation devices.
One particular spinal fixation technique includes immobilizing the spine by using orthopedic rods, commonly referred to as spine rods, that run generally parallel to the spine. This is accomplished by exposing the spine posteriorly and fastening bone screws to the pedicles of the appropriate vertebrae. The pedicle screws are generally placed two per vertebra, one at each pedicle on either side of the spinous process, and serve as anchor points for the spine rods. Clamping elements adapted for receiving a spine rod therethrough are then used to join the spine rods to the screws. The aligning influence of the rods forces the spine to conform to a more desirable shape. In certain instances, the spine rods may be bent to achieve the desired curvature of the spinal column.
U.S. Pat. No. 5,129,388 to Vignaud et al. discloses a spinal fixation device including a pedicle screw having a U-shaped head rigidly connected to the screw. The U-shaped head includes a U-shaped channel for receiving a spine rod therein. The U-shaped head is internally threaded so that a set screw having external threads may be screwed therein. After the pedicle screw has been inserted into bone and the spine rod is positioned in the U-shaped channel, the set screw is threaded into the internal threads of the coupling element for securing the spine rod in the U-shaped channel and blocking relative movement between the spine rod and the pedicle screw. The fixation device also includes a cap covering an upper portion of the U-shaped element to prevent the arms of the U-shaped element from spreading upon threading the set screw into the U-shaped head.
Surgeons have frequently encountered considerable difficulty when attempting to insert spinal fixation devices such as those disclosed in the above-mentioned ""388 patent. For example, surgeons have frequently been unable to efficiently and adequately place the spine rod into the U-shaped heads of the bone screws. This is because the U-shaped heads of the screws are often not aligned with one another due to curvature in spines and the different orientations of the pedicles being instrumented. The spine rods are often bent in multiple planes in order to couple the pedicle screws to the rod, which may lead to weaker connections with the rod. These problems also result in significantly longer operations, thereby increasing the likelihood of complications associated with surgery.
In response to the problems noted in the ""388 patent, U.S. Pat. No. 5,733,286 to Errico et al., U.S. Pat. No. 5,672,176 to Biedermann et al., and U.S. Pat. No. 5,476,464 to Metz-Stavenhagen disclose polyaxial spinal fixation devices wherein the anchoring element fixed to the bone has a spherically-shaped head. The fixation devices in the subject patents also have orthopedic rod capturing assemblies for securing orthopedic rods in the capturing assemblies and connecting the rods with the anchoring elements. The spherically shaped heads of the anchoring elements permit movement of the anchoring elements relative to the orthopedic rod capturing assemblies. However, the above-mentioned patents do not solve all of the deficiencies noted in the Vignaud ""388 patent because the respective spinal fixation devices may shift following insertion. This is due primarily to the fact that there is insufficient surface area contact between the spherically-shaped heads of the anchoring elements and the rod capturing assemblies. In addition, the devices are complex and are difficult to manufacture.
Other polyaxial bone fixation devices are taught in International Patent Publication WO 88/03781. This publication describes a device for securing bone screws in openings in an osteosynthesis plate. The head of the bone screw is provided with longitudinal grooves that enable the head to expand after being positioned in one of the openings in the plate. After a screw has been inserted through one of the openings, an adjusting screw is threaded into a bore at the top of the head for expanding the head. The outer surface of the expanded head presses against the opening in the plate so as to secure the screw to the plate. U.S. Pat. No 4,484,570 also discloses a bone screw used in conjunction with a plate wherein the screw has a head with a central opening and slots that subdivide the head into tongues. An expander having a conical surface is inserted into the central opening of the head for expanding the head and locking the screw in relation to the plate. However, the WO 88/03781 and U.S. Pat. No. 4,484,570 references concern bone plates and not spinal fixation devices using orthopedic rods.
In spite of the above-mentioned devices, there remains room for improvement of prior art spinal fixation devices in the manner of locking the screw head, the complexity of use, difficulty in properly positioning the orthopedic rod and the rod-capturing assemblies, the required manipulation of the many parts associated with some complex devices and post-operative movement of the rod-capturing assemblies relative to the bone anchoring elements due to the weak interfaces between the two.
In accordance with certain preferred embodiments of the present invention, a pedicle screw assembly includes a fastener having a tip end for insertion into bone and an expandable head at the opposite end thereof. The head may be expandable by virtue of the material of which it is made or its design. One preferred design for an expandable head includes a head having one or more slots formed therein. In this embodiment, the expandable head preferably has an outer surface including a convex portion, a recess defining an inner surface having an inner dimension, and at least one slot extending between the inner and outer surfaces thereof for allowing expansion of the head. The assembly also includes an insert that can be positioned at least partially in the recess of the head. The insert desirably has an outer surface including an outer dimension that is greater than the inner dimension of the recess. In certain embodiments, the insert may be rotatable with the recess of the head.
The pedicle screw assembly also has a coupling element for coupling the pedicle screw to an orthopedic rod. The coupling element preferably includes a bore extending therethrough for receiving the fastener, and a seat for receiving the head of the fastener. The seat may include a concave portion for receiving the convex portion of the head and for allowing the fastener and the coupling element to pivot relative to one another before being locked to prevent further pivotal movement. The assembly may also include a locking element associated with the coupling element for locking the orthopedic rod in the coupling element after the rod has been positioned therein. The locking element is desirably adapted for forcing the insert into the recess of the head so that the outer dimension of the insert bears against the inner dimension of the head, whereby the head expands against the seat of the coupling element for preventing further pivotal movement of the coupling element relative to the fastener.
The expandable head of the fastener provides for an improved level of surface contact between the head and the seat of the coupling element. This increased level of surface contact results in a more secure locking force being generated. In addition, the locking force is increased by the friction force, which acts at the interface between the head of the fastener and the coupling element. This friction force is proportional to the normal force of the head of the fastener on the seat of the coupling element and thus increases as the insert acts to expand the head of the fastener. As a result, the likelihood of post-operative shifting and/or movement of a spine rod or coupling element relative to one or more of the bone fasteners is significantly reduced. Thus, the present invention provides for a more reliable spinal fixation device and overcomes the post-operative shifting problems seen in prior art devices. Moreover, the pedicle screw assembly of the present invention has fewer parts. As a result, implantation operations are greatly simplified and the possibility of a component being dropped inside a patient""s body greatly reduced.
In certain preferred embodiments, the fastener is a pedicle bone screw having external threads extending from a tip end of the screw toward the head. The fastener may have one or more holes in the threaded portion therein for receiving bone graft material as disclosed in U.S. Pat. No. 4,484,570 to Sutter. Instead of using a screw for securing the screw to bone, in other preferred embodiments the fastener may includes a hook-shaped anchoring element as disclosed in above-mentioned U.S. Pat. No. 5,476,464 to Metz-Stavenhagen. The fastener may also be a structure having barbs on an outer surface thereof, whereby the fastener is forced into bone and the barbs prevent the fastener from being withdrawn from the bone. The fastener preferably includes a neck between the head and the tip end, the neck desirably being located adjacent the head. The neck includes a concave surface and has a diameter that is generally smaller than the diameter of the threaded portion of the fastener. The head of the fastener preferably includes a recess defining an inner surface having an inner dimension. The head also includes a plurality of slots extending between the inner and outer surfaces of the head. The fastener preferably has a longitudinal axis extending from the tip end to the head with the recess and the inner and outer surfaces thereof being centered on the longitudinal axis. The slots generally commence at the top surface of the head and extend toward the tip end of the fastener in directions substantially parallel to the longitudinal axis. The plurality of slots generally subdivide the head into two or more flexible arms at the upper end of the fastener. In one preferred embodiment, the head has six slots formed therein for subdividing the head into six flexible arms. The flexible arms are desirably adapted for flexing away from the longitudinal axis for expanding the outer surface of the head. The flexible arms are also capable of flexing toward the longitudinal axis for compressing the head, so as to reduce the outer dimension of the head and allow for assembly of the fastener into the coupling element.
The expandable head also preferably includes at least one tab extending into the recess for at least partially securing the insert in the recess. The tabs are preferably formed at upper ends of the flexible arms, whereby each flexible arm desirably includes one tab. The flexible arms are generally arranged in a substantially annular configuration around the circumference of the recess with the tabs extending into the recess for securing the insert.
The insert preferably has an upper end and a lower end and a longitudinal axis extending between the upper and lower ends. The insert also preferably has an outer surface and a flange extending around the outer surface, between the upper and lower ends thereof. In preferred embodiments, the flange lies in a plane substantially perpendicular to the longitudinal axis of the insert. The flange preferably defines the outer dimension of the insert, i.e., the section of the insert having the largest diameter. In other words, the flange provides the largest diameter portion of the insert. When viewed from the side or in a cross-sectional side view, the outer surface of the insert tapers inwardly from the flange toward the lower end thereof. In certain embodiments, the lower end of the insert is substantially spherical in shape. However, in other preferred embodiments, the lower end of the insert is substantially flat. The upper end of the insert preferably has a radial surface that is adapted for engaging the orthopedic rod. The radial surface of the upper end insures good surface contact between the rod and the insert, regardless of the angle of the insert relative to the rod. When the insert has been positioned at least partially in the recess of the expandable head, the upper end of the insert extends beyond the top of the head for, inter alia, preventing a rod from contacting the head of the fastener.
In other preferred embodiments, the insert includes at least one radial projection, and preferably a plurality of radial projections extending outwardly from the periphery of the insert. The radial projections are preferably provided at the upper end of the insert, whereby each projection is sized to extend into one of the slots formed in the expandable head when the insert is at least partially positioned in the recess. The insert has a radial flange preferably located immediately below the radial projections and a socket formed at the upper end thereof. The socket is adapted for receiving a driver, such as a screwdriver, a hexagonal wrench or the like. In this embodiment, the fastener is attached to bone by inserting a driver into the socket of the insert, and then turning the driver to rotate the insert in either a clockwise or counterclockwise direction. In turn, the insert transmits the driving torque from the driver to the fastener. The driving torque is transmitted from the insert to the fastener at the radial projection/slot interface.
In other preferred embodiments the insert does not have a socket, but has an axial bore extending therethrough. In these particular embodiments, the recessed portion of the head has a socket formed therein that is sized to receive the driver. When the insert is positioned at least partially in the recess, the bore extending through the insert is substantially aligned with the socket formed in the head so that a driver can be passed through the bore to reach the socket. The driver is then rotated to rotate the fastener in either a clockwise or counterclockwise direction for anchoring the fastener in the bone.
The coupling element preferably includes a substantially U-shaped element having an upper end and a lower end and a longitudinal axis extending between the upper and lower ends. The coupling element has an interior surface defining a bore extending in a direction substantially parallel to the longitudinal axis thereof and an exterior surface. The interior surface of the coupling element defines the shape of the bore that receives the fastener. In certain embodiments, the bore extends in a direction substantially parallel to the longitudinal axis of the coupling element. The coupling element preferably includes threads adapted for being threadably engaged with the locking element. In one embodiment, the threads are formed on the interior surface of the coupling element and the locking element includes a set screw having external threads for threadably engaging the internal threads of the coupling element. However, in other preferred embodiments, the threads may be formed on the external surface of the coupling element, whereby the locking element includes a locking nut having internal threads adapted for threadably engaging the external surface threads of the coupling element. The coupling element may also have one or more impressions or grooves formed therein for receiving a controlling device, such as a persuader instrument for seating the rod in the coupling element. In some embodiments, the impressions or grooves generally extend in a direction substantially perpendicular to the longitudinal axis of the coupling element.
The interior surface of the coupling element at the lower end thereof preferably defines the seat including a concave portion for receiving the convex portion of the head and allowing the head to pivot relative to the coupling element before being locked in place. The seat is preferably provided adjacent the lower end of the coupling element in an expansion cavity. The expansion cavity preferably has a diameter that is larger than the diameter of the internal threads.
During assembly of the above-mentioned pedicle screw device, the tip end of the fastener is passed through the bore of the coupling element until the expandable head is positioned within the expansion cavity, with the convex outer surface of the head in contact with the concave seat of the coupling element. In certain preferred embodiments, this is accomplished by passing the tip end of the fastener through the upper end of the coupling element toward the lower end thereof. As the tip end and the threaded portion of the fastener passes toward the lower end of the coupling element, the head must be compressed so that it may pass by a small diameter portion of the bore of the coupling element because the outer surface of the head has a diameter that is greater than the smallest diameter of the bore. In one preferred embodiment, the smallest diameter portion of the bore is defined by the internal threads of the coupling element. However, in other embodiments, the smallest diameter portion may be an inwardly extending projection, such as an annular ring. As a result, the flexible arms of the head must flex inwardly for compressing the head, whereupon the head may pass through the small diameter portion of the coupling element. The head continues to pass through the coupling element in a compressed state until the head reaches the expansion cavity, whereupon the flexible arms are free to return to their original, unflexed position.
Once the insert has been positioned in the recess, the flexible arms are incapable of flexing inwardly because the outer dimension of the insert abuts against the inner dimension of the recessed portion of the head. As such, the head can no longer be compressed. Moreover, the fastener cannot be removed from the coupling element as long as the insert remains secured in the recessed portion of the head.
During a spinal fixation operation, after the fastener has been screwed into bone, the coupling element is free to pivot relative to the fastener and the insert secured thereto. The neck portion of the fastener, preferably having a concave surface with a diameter less than the diameter of the threaded portion of the fastener, enables the coupling element to pivot through a broader range of angles relative to the longitudinal axis of the fastener. Thus, a spine rod may be more easily positioned within the rod receiving opening of the coupling element. After the rod has been positioned within the rod receiving opening, the rod is then locked in place by threading the locking element into the threads of the coupling element. As the locking element tightens down upon the rod, the rod, in turn, exerts a downward force on the insert. In turn, the insert moves further into the recess for forcing the flexible arms to move away from one another for expanding the head of the fastener. As the head expands, an increased area of the head engages the seat of the coupling element and the normal force the head exerts on the seat increases. The friction force acting along the interface between the head and the coupling element therefore also increases, since it is proportional to the normal force. The enhanced surface area contact between the head and the coupling element and the higher friction force improve the locking force therebetween and prevents further pivotal movement of the coupling element relative to the fastener. As a result, the likelihood of post-operative shifting and/or moving of the pedicle screw assembly is greatly reduced, thereby minimizing the occurrence of post-operative complications for spinal implant patients. In certain preferred embodiments of the present invention, the coupling element secures the orthopedic rod directly above the head of the fastener. However, in other preferred embodiments, the coupling element may be structured to hold the orthopedic rod offset from the longitudinal axis of the pedicle screw as disclosed in U.S. Pat. No. 5,344,422 to Frigg and U.S. Pat. No. 5,584,831 to McKay.
The present invention also preferably includes a tool for securing the fastener in bone. The tool is preferably a driver having a rotatable shaft and one or more prongs extending from an end of the shaft for engaging the slots. In preferred embodiments the driver has one prong for each slot in the head of the fastener. The driver may also have external threads at a lower end of the shaft. The external threads are preferably adapted for engaging the internal threads of the coupling element when a fastener is being driven into bone. The engagement of the external threads of the driver and the internal threads of the coupling element generally stabilizes the pedicle screw assembly when the fastener is secured to bone. Specifically, the engagement of the threads prevents the coupling element from moving relative to the fastener when driving the fastener into bone, thereby simplifying installation of the fasteners.
These and other objects, features and advantages of the present invention will be more readily apparent from the detailed description of preferred embodiments set forth below, taken in conjunction with the accompanying drawings.